Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is very common to employ a raster output scanner (ROS) as a source of signals to be imaged on a photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off as it moves, or scans, across the photoreceptor, to form the desired image on the photoreceptor. A common technique for effecting this scanning across the photoreceptor is to employ a rotating polygon surface; the laser beam from the ROS is reflected off the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor.
FIG. 1 shows the well-known basic configuration of a scanning system used, for example, in an electrophotographic printer or facsimile machine. A laser source 10 produces a collimated laser beam 12 which is reflected from the facets of a rotating polygon 14. The polygon surface 14 deflects the laser beam 12 to direct a line of beam scanning 16 toward the photoreceptor 18. When the line of beam scanning 16 reaches photoreceptor 18, a scanning line 20 results across photoreceptor 18. In FIG. 1, the photoreceptor 18 is shown as a rotating drum, but those skilled in the art will recognize that this general principle, and indeed the entire invention described herein, is applicable to situations where in the photoreceptor is a flat plate, a moving belt, or any other configuration. The periodic scanning of beam 16 across the rotating receptor 18 creates a raster, or array of scan lines, on the photoreceptor 18, creating the desired image to be printed. In real-world situations, such a configuration will typically further include any number of lenses and mirrors to accommodate a specific design. Unavoidable imprecisions in the shape and/or mounting of these optical elements will inevitably introduce certain anomalies in the quality of the scan line on the photoreceptor, and consequently create flaws in the printed document. Two important types of such anomalies are "skew" and "bow."
Skew is the error in rotational orientation of scan lines relative to the photoreceptor. As shown in FIG. 1, the scan line 20 is rotated slightly relative to a line 22, which is parallel with the axis of the photoreceptor 18. If the photoreceptor is a plate or belt, the scan line 20 may be skewed relative to an important base line, such as a line perpendicular to the edge of the belt. Further, if a number of rasters are superimposed on a document, as in a color copier, the different skews of the different rasters will cause a noticeable interference pattern on the document, to the great detriment of copy quality.
Bow is the quality of a scan line to form not a straight line on the photoreceptor, but a line which bows about a central midpoint. An example of bow is shown by scan line 20' in FIG. 1B. Even in a monochromatic printer, a pronounced bow of the lines in a raster will be noticeable. In a color printer or copier, the different extent and/or direction of bow for each superimposed color raster can be an important cause of a conspicuous color banding on the document.
Depending on the types of imprecision in the construction of the apparatus, the bow may bend in either direction relative to the center line 22. In manufacturing situations, it is also very common to have both skew and bow simultaneously evident in the scan line 20.
It is an object of the present invention to provide a simple means for correcting skew and bow in a scanning system.
Another object of the present invention is to provide such means which may be implemented as needed in individual electrophotographic printers as part of the manufacturing process.
Another object is to provide such means which may be simply adjusted as needed for individual electrophotgraphic printers.
Another object is to provide such means which may be constructed from simple and well-known parts.
Other objects will appear hereinafter.